NEW MORPHOMETRIC METHODS TO QUANTIFY COMPLEX AMMONOID SUTURES

Systematic taxonomy of the ammonoid family Cyclolobidae is complicated due to the high degree of complexity and intraspecific variation in sutural patterns. Recently, the authors described a new collection of ammonoids from the Permian of West Texas, and these new data suggest that a taxonomic revision of Cyclolobidae is needed.

Morphometric methods have historically been an effective tool in taxonomic analyses of ammonoids, although their application has primarily been restricted to the quantification of conch dimensions. We introduce several morphometric methods and indices to quantify sutural elements. These new methods involve angular measurements of various suture components in relation to the axial plane of planispiral forms, as well as linear measurements between specific suture components.

Resultant data were compared to conch dimensions, and it was found that along ontogenetic progressions, some of these sutural indices correlate strongly with overall conch diameter. Therefore, these relationships can be used to reasonably estimate the total diameter of incomplete specimens at an ontogenetic stage concurrent with that of the measured suture. Additionally, some of these sutural indices correlate with one another through ontogenetic progression, and therefore only one of several combinations of sutural components is necessary to estimate some conch dimensions.

We suggest that these new morphometric methods can provide a more accurate systematic taxonomy for ammonoids with complex sutures, and that more robust datasets may be produced from relatively incomplete specimens. We further suggest that these suture-based morphometrics can be useful for taxonomic studies of other ammonoid families characterized by complex suture patterns.